CHAPTER VII. 



RAINSTORMS. 



The injurious effects of beating rains on the soil have already been 

 mentioned. They pound the upper surface down or cover it with great 

 quantities of silt. The immediate result is oxygen starvation for the roots. 

 The mechanical effect of heavy rains on the plant itself is first to be con- 

 sidered. There are many natural devices in plants which safeguard the 

 leaves from the beating and tearing effects of heavy rains or the undue 

 accumulation of water from long continued gentle rains. Stahl^ and 

 Jungner- have given a thorough presentation of these conditions and call 

 attention to the formation of the tips and to the position and repeated 

 division of the leaf surface, etc. 



The direct results of the rain are a decrease of transpiration and a great 

 water absorption by the roots. They have been less considered. Here also 

 the swelling of the wood of trees belongs. Friedrich's investigations^ show 

 that a constant swelling of the tree trunk (aside from any direct growth) 

 takes place during the night because with lessened transpiration, the tree 

 swells, while in the daytime it shrinks. The differences will be most marked 

 when the growth is rapid and the wood swells, especially when rain comes 

 after considerable drought. Bark and periderm are less affected. Growth 

 and swelling of the wood cylinder are regulated by the influence of atmos- 

 pheric humidity on the tops of the trees. 



It is thus easily evident that smooth bark will crack in places because 

 of the strong and sudden increase in swelling and growth. When the soil 

 is rich and the atmospheric humidity great, these cracks may become open 

 wounds, constantly increasing by bacterial infection. Rough places then 

 arise on the bark of the young tree trunks. These may be observed, for 

 example, in lindens, elms, ashes, maples, etc., near wet ditches and ponds. 



The influence of longer periods of rain manifests itself in herbaceous 

 plants, even more than in woody ones, by cracks in fruit and stems. Among 



1 Stahl. B., Regenfall und Blattgestalt. Ein Beitrag- zur Pflanzenbiologie. 

 Annal. de Buitenzorg.; cit. Bot. Jahresber. 1893, 1, p. 49. 



- .Jungner, J. R., Om regnblad, daggblad och snoblad. Bot. Not.; cit. Botan. 

 Jahresber. 1893, p. 49. 



3 Freidrich, Josef, Uber den Einfluss der Witterung auf den Baumzuwachs. 

 Mitteil. lib. d. forstl. Versuchswesen Osterreichs. Wein 1897, Part 22. 



